Fixed cloning SkSL ReturnStatements without expressions
Fixed cloning SkSL ReturnStatements without expressions

Previously, cloning such a node would have dereferenced a null pointer.

Change-Id: Id2f8248b3bbe3233f14e93d1383c95ee45ad6fc7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/496376
Reviewed-by: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: Ethan Nicholas <d2cfb8c2f603266f4dc3a6f09acbcc0089603b37@google.com>

google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia